Protons are probably smaller than long thought
uni-bonn.de
uni-bonn.de
I’ll leave it as an exercise for the reader to decide whether the title of the article is clickbait.
I've been to oodles of talks on the problem. It has had special sessions at APS meetings and I've watched acquaintances and colleagues expend meaningful fractions of their careers attempting to resolve the Proton Radius Puzzle.
If an appeal to authority is required -- Nature gave it the cover. https://www.psi.ch/en/media/our-research/protons-smaller-tha...
Pohl's measurements were incontrovertible, yet they disagreed with decades of work. It has been a big problem in a quiet community for some time.
If someone has a reliable angle to resolve the problem, it is big news. Enough really good physicists have tried and failed to resolve the conundrum that one should wait to see if this new approach holds up, too.
Additionally, I’m not sure why you’re saying we should wait to see whether the results hold up, if you don’t think writing “probably” in the headline is clickbait.
My definition of clickbait is a little different than that. It’s not so much the newsworthiness of the item behind the headline (one man’s old news is another man’s new insight), but rather the form of the headline itself. The more the headline plays on my emotions to pull me in, to try and push its relevancy ahead of an otherwise objective cataloging of information, the more clickbaity it is (to me at least).
This particular headline is far from the worst, but the air of mystery it elicits seems to waft “psst, you gotta check this out.” So imho, it is slightly clickbaity.
(I was going to use an analogy, but a previous HN article today taught me it’s better to use those in non-debate contexts).
It was actually off, but anybody who did experiments that disagreed with Millikan's "adjusted" things a little bit to match better.
So, it took a while for the value of the charge of the electron to "settle" to where it belonged.
Scientists are people--with all the same failings.
I read the wikipedia articles for "proton" and "proton radius puzzle" and the press release we're commenting on. I didn't notice any mention of a theoretical prediction of proton radius - only some older experiments suggesting the larger size, and some newer experiments suggesting the smaller size.
Was there ever a theoretical prediction?
I feel like I'm missing something fundamental here, and I'd like to know what it is.
Meanwhile we can measure the fine structure constant to 12 decimal places (https://en.wikipedia.org/wiki/Electromagnetic_coupling_const...) and that measurement is in very close accord with the predictions of quantum mechanics.
One is to measure the g-factor of the electron in a Penning trap. This can be related to the fine structure constant using quantum electrodynamics (QED).
The other is to measure the recoil that an atom receives when it absorbs a photon in an atom interferometer. By combining the result with another well known constant, the fine structure constant can be calculated.
The results of both methods agree to 12 digits, which shows that the QED calculations of the electron g-factor are correct on that level.
Defining the radius anything more than extremely statistically runs up against the uncertainty principle of all these constituent particles.
The amazing accuracy you have read about is the anomalous magnetic moment of the electron, which is a very clear cut measurement.
From reading the article it seems like there's a hard distance boundary beyond which it will not "collide" with the electron?
Just a layman but I think they usually define size via the halfway distance between the centers of two identical bound particles. Not entirely sure what that would be in this case though, given helium-2 is unstable and helium-3 might give a different result. (?)
I don't know the exact details of the definition though.
Now, is there a reason why there isn't a something else that acts like a proton?
That gets into Elementary particle physics and what combinations are stable and have matching charge. Quarks have charge +- n/3 and generally come in triples.
Could the number of gluons vary? Maybe? But they wouldn't affect the size measurements much? And variations wouldn't be stable?
This has important implications.
Consider a quantum mechanics experiment where you emit a proton at A and try to detect it at A′. You will find that that there is some quantifiable chance of detecting the particle at A′ some time T. Call that probability P.
Now consider a second experiment where you emit a proton at both A and B, and try to detect them at A′ and B′. What is the probability of finding a proton at A′ at time T? You will find that you get a different number! This is because the proton at A could travel to A′ and be detected, but also the proton at B could travel to A′ and be detected too. Since you cannot distinguish the two protons, you won’t be able to distinguish between these two outcomes, and so the probability must be different from P.
Couldn't it be that your measurement for a proton being at A' is simply measuring the wrong feature of the protons?
edit: I have no idea if protons are indistinguishable from one another, but this experiment doesn't seem compelling.
For protons, they may come with a serial number, but we're not allowed to read it because of the constraints built into God's programming language.
Protons don't behave like cats.
In my statistical mechanics course, we went through an illuminating exercise where we started by trying to take account of every atom in a gas cloud. We started taking limits and making assumptions. One of them was that all atoms are indistinguishable from each other. This decreases the possible states of the system by N! (factorial, not surprise). After making that assumption, out pops the ideal gas law.
It's all totally, perfectly self consistent, but it does not derive from first principles like set theory or mathematical logic do. Physics is an experimental science; they are not required to state their axioms. Oftentimes they do (QFT for example), but the most glaring case where they don't is anything involving information.
The whole postulate that information is physical is something that was stumbled upon, and then turned out to explain a whole bunch of other weird things like heat and entropy, and some of those explanations in turn implied that information is physical.
I suspect that our current efforts to build cryptographically-relevant quantum computers are a lot like the efforts to build perpetual motion machines in the 1700s. Our current understanding of things isn't wrong, but there is some undiscovered general principle that we keep butting up against, so we'll keep trying to build these things until we figure out why nature keeps blocking us. That discovery -- rather than a computationally-useful device -- will be the most important result of all the quantum computing research going on right now.
A quantum computer only behaves like our mathematical ideal quantum computer if it's sufficiently isolated from the rest of the universe: otherwise you don't get the “indistinguishable states” thing and the amplitudes don't sum, and the computer stops computing and starts being a regular ol' physics experiment.
It's an engineering problem, as far as I understand. Get things cold enough, get things isolated enough, so they stay entangled with each other and not with the rest of the universe.
The temperatures are easy enough; you just have to compensate for thermal noise. The isolation isn't; without isolation, your signal is wrong.
With a quantum computer, you can only do that at the end of the computation. Not half-way through; that'll cause the computer to start doing a different (unwanted) computation instead. While the calculation is happening, you need (a high probability of) total isolation from the rest of the universe, so that the intermediate state of the computer only interferes with itself.
> So, I am not convinced having a deeply isolated part of the universe is the answer; in fact, the isolation vs. signal quality tradeoff makes it sound more and more like a fundamental limitation of practical concern.
It is a fundamental limitation of practical concern! Just like the need to keep conventional computer processors cool or they melt, or the fundamental limitations on the bandwidth that a radio frequency can give you. The people who deal with these limitations are called engineers.
That is far from being clear.
People in the 1700s were totally convinced that building a perpetual motion machine was just "an engineering problem".
If this hidden variable has no effect at all, then it is useless. Most physicists don’t care to include extra variables in a quantum mechanical theory that have no effect at all. By definition, they could not be measured, and so they would be extra baggage to carry around to no effect.
Physicists don’t much like nonlocal theories either. Any theory that requires information about the particle at A to travel instantaneously to B in order to change the state of the particle there is going to be hard to sell. You might have heard of a fellow called Einstein, who proved that nothing can go faster than the speed of light.
Thus, most physicists take the easier road, as it requires only that subatomic particles are indistinguishable. All this means is that particles are too simple to be uniquely identified. You can in principle tell the difference between two baseballs, because they have different patterns of wear and other markings on their surface. But those wear patterns and markings are formed out of the complex arrangements of trillions or quadrillions of atoms. It is easy to see how rearranging the ink molecules on the surface of a baseball could create a unique baseball, or how selectively removing molecules from the surface of the leather (by scratching it, for example) could do the same.
But subatomic particles are too simple to have that kind of internal state. Even atoms are only slightly distinguishable. Most carbon atoms have 12 electrons, but a carbon ion might have 11 or 13 electrons. You can distinguish between the atom and the ion, but not between two atoms or two ions.
In quantum mechanics, probabilities are given by the square of the absolute value of more fundamental quantities called amplitudes. When something happens in two ways that can be distinguished, you must add the probabilities. When something happens in two indistinguishable ways, you must add the amplitudes, which yields a different probability after squaring. For example, .3^2+.4^2 != (.3+.4)^2. Thus, you can verify experimentally whether particles are or are not distinguishable.
Thank you for a terrific explanation. Could you please go one layer deeper? Why does whether probabilities or amplitudes are summed imply fungibility (or its absence)?
If swapping two things around gives a result indistinguishable from not swapping them, that's fungibility.
Before I say anything, if you have never heard of amplitudes before, you should read the Feynman lectures on physics vol. III, which you can find here: https://www.feynmanlectures.caltech.edu/III_toc.html Specifically read chapter 1 and chapter 3. The specific situation about distinguishable/indistinguishable particles is described in section 3-4.
Your question is kind of phrased backwards. Probabilities and amplitudes are human inventions to describe Nature's behavior, so by themselves they don't imply anything about Nature. The implication is the other way around: Nature has decided that some particle pairs are indistinguishable (proton vs. proton) and some are distinguishable (proton vs. neutron). Different rules apply to the two cases.
This indistinguishability is a pure quantum phenomenon. In a classical world, all objects are distinguishable---you can in principle label all protons and know which is which. But Nature does not work like that, and there exists this peculiar notion that you cannot tell two protons apart not even in principle. There is no deeper explanation of this phenomenon AFAIK---it is what it is.
You can tell experimentally whether two particles are or are not distinguishable by running the experiment as in Feynman 3-4. If you observe a distribution consistent with the add-amplitude rule, then the particles are indistinguishable. Any attempt to distinguish them leads to the contradictions explained in Chapter 1.
There is an even more peculiar phenomenon. Despite being indistinguishable, swapping two indistinguishable particles is not a no-op because it changes the amplitudes. You must still add amplitudes, but the amplitudes are different, yet different in such a subtle way that you still cannot tell the particles apart. Feynman 3-4 tells you how the amplitudes change during the swap, with a deeper explanation in chapter 4.
The statement that protons are indistinguishable is not strictly correct either, because protons have a spin. Protons with the same spin are indistinguishable, but you can tell apart protons with different spin. The spin of protons can only assume two values, so effectively there are two classes of protons, indistinguishable within the class.
In your specific case, it is clearly false that the probability of having one particle in one place is 200%. However, my statement still holds for expectations, and you end up with an expected two particles in one place. In the indistinguishable case, you must compute expectations based on amplitudes, not probabilities.
Going back to the experiment that I described, you can imagine that the particles are released at A and B with opposite spins, and then the detector at A’ only detects the spin that corresponds to the particle at A. This causes you to measure yet another probability, distinct from the other two, because there are now more possibilities and there are still multiple ways to cause the detector to find something. It could detect the proton from A, but the proton from A could also have its spin flipped and thus not be detected. The particle from B could arrive at A’ with the wrong spin and not be counted, or it could have its spin flipped along the way and be counted. You still cannot tell which proton you detected!
Similar complications occur with polarization of photons, which someone else mentioned in one of the comments. It’s worse though because polarization is a continuous quantity, and there are more ways to change it.
Incidentally, amplitudes are actually complex numbers. You can think of them as little arrows, like this: →, or this: ↖. In fact, these arrows are also rotating with the passage of time; they trace out little circles. To calculate the probability, we square the absolute value of the complex number. The absolute value of a complex number is equal to the length of the arrow, and squaring a length gives you an area. Thus the probability is essentially the same as the area of the circle traced out by the rotating arrow.
Events with high probability correspond to long arrows (big amplitudes), and low probability events have short arrows (small amplitudes). Amplitudes can cancel out when added together if they point in opposite directions. Thus we observe that some sequences of events have very low probability. We sometimes say that these events interfere with each other.
Sometimes this interference seems mysterious, as in the double–slit experiment, and other times it seems very mundane. In real life we rarely bother to calculate the probability that the batter will hit the ball before the pitcher throws it, but calculating the correct answers in quantum mechanics requires taking into account many such unusual events.
One interesting thing about protons is that you can describe the effect of exchanging any two of them in precise terms, and end up with macroscopic predictions that can be tested. So you're not limited to just trying to measure every proton in a bag to see if they're all the same. There are other ways to test the hypothesis.
Not if they interfere in just the right way to keep the probability the same as in P.
I see a second problem with this experiment - in the double slit experiment, there is one electron interfering with itself. If you released two protons at the same time, they'd interact with each other and change the probabilities, even classically.
As a fun consequence, the theory treats all particles the same -- even so-called quasi-particles! The difference between fundamental and other kinds of particles is that the fundamental particles can exist (at least for a short time before decaying) in vacuo.
Isn't a sigma a lot? Like, I think that's a standard deviation? If there is even a longshot chance that we are might be off on that constant by multiple standard deviations, isn't that certainly a less-determined constant than, say, the acceleration of gravity? I feel like there is no chance in hell we could one day discover that our calculations are that far off for gravity.
EDIT: distance, not size
If it's worth correcting people, it's worth correcting them correctly... Don't you agree?
Also... The previous poster was pretty obviously talking about the constant of Gravitation (https://en.m.wikipedia.org/wiki/Gravitational_constant). His way of phrasing it is a common English shorthand that I see frequently enough that it's a well understood usage. They may not have used precise language, but their phrasing was definitely less misleading than your (incorrect) correction.
You’re right that it’s distance, of course, but I was coming from a place of trying to be helpful, and thought that if they were talking about terrestrial physics, it’d be easiest to imply it depended on the size of the earth (which determines our distance from it) and your height.
I think you were trying to score points, and get an ego fix by correcting someone else.
Not so much fun when someone else dunks on you, though, is it?
The mass of the accelerating object is absolutely a variable.
The standard acceleration of gravity is, btw, defined, so no uncertainty. The gravitational constant G is known only to 10^-5 or so.
What I mean is, just like the sun radiates in the EM spectrum and tells us a lot about the properties of those fields, does gravity do something similar?
The latter says "The new result implies that earlier attempts to measure the proton’s radius in electronic hydrogen tended to overshoot the true value. It’s unclear why this would be so" and it seems that these researchers have now shown why.
As scientists we want to know if our understanding of nature is correct. To test this, we measure the same quantity, for example the proton charge radius, in different ways. If the underlying theory is correct, the results should agree within the experimental uncertainties.
Since 2010 there was a big disagreement between the proton charge radii measured by hydrogen spectroscopy and electron/proton scattering (which roughly agreed at the time), and a much more accurate measurement using muonic hydrogen spectroscopy. This has lead to a lot of excitement, since discrepancies could be a hint for new physics. Since then, more accurate hydrogen spectroscopy experiments have been performed and most agree with the muonic hydrogen value. This probably indicates that the discrepancy is due to underestimated error bars in the old measurements.
In contrast to laser spectroscopy which gives relatively direct results, getting the charge radius out of electron scattering data is notoriously hard. Different groups have found different charge radii from the same data for a long time.
The most unsettling feeling. I often have that feeling when doing the devops and infrastructure part of my job, e.g. encountering paradoxes in the dark dreary bowels of systemd.
The idealist and perfectionist in you wants to keep digging deeper to arrive at a proper understanding. The realist and lazy SOB in you wants to slowly back away and pretend this never happened. This dialectic hopefully guides you toward a happy compromise, somewhere down the middle.
This week, we had such a failure when testing a change. The root cause was found eventually, but what two extremely good engineers plus me still could not figure out was why that problem was only surfacing now, after years of having that buggy code in there. Very important code. I did not want to risk some obviously unknown property of it be our demise later on.
Fortunately, in this case it just turned out that the seemingly unrelated changes did, after all, hold the now failing thing differently than before. As it had been used, the problem was masked entirely, and the code did work well for all those years.
Thanks in advance to anyone willing to take the time to give me an ELI5 on this :)
Assuming ducks stop the bullets.
And assuming it's not just one immortal duck flying around super fast.
For the proton, the "size" is defined as a length-valued parameter in the function that expresses the charge distribution through space. The parameter is objective - but its association with the word "size" is imprecise. It is necessarily imprecise, not because of anything quantum or even unfamiliar, but because there's not another English word for the scale of objects without hard edges.
We're at quantum scales and the very notion of "size" is rather ill-defined.
The position of a proton, is - as a matter of fact - a probabilistic affair, so when you talk about size ... if you don't even know where the darn thing is, how are you going to measure where it starts and where it ends ...
I need to spend less time on Twitter.
So, I'll be sitting down for a little while.
And that was all.
Or possibly strings. If that’s not true, the best answer is the tautology: quarks are made of quarks.
Analogy: there are no "red pixels", there are just red excitations in the display RGB field.